In recent years, the internal power supplies of computers and other electrical and electronic equipment have become smaller, of lower weight, and less costly. In many cases, such power supplies have been designed so that they do not require a transformer. Equipment incorporating a power supply without a transformer typically has an input circuit consisting of an input bridge rectifier which produces direct current from the alternating current (AC) input power. Such bridge circuits typically employ a capacitive input filter. Unfortunately, the capacitive input filter and the rectifier circuit present a poor power factor load which is reflected back to the AC power lines.
Recently, computers and other products having transformerless internal power supplies have been introduced which include power factor correction circuits; these circuits change the capacitive power factor of the transformerless power supply of such equipment to approximately unity power factor. While these power factor correcting circuits thus have reduced the power factor problems of equipment having transformerless power supplies, they have in turn introduced a new set of problems, particularly where the power factor corrected load is not connected directly to the AC power mains but is supplied with power through an intermediate AC power supply system. These power supply systems can include linear transformers used for isolation or voltage change, ferroresonant transformers, uninterruptible and standby power supplies, DC to AC inverters, line conditioners, as well as AC generators and the like. Uninterruptible power supplies (UPS), standby power supplies (SPS), and line conditioners are now widely used to protect the power supplied to critical loads, such as computers, emergency lighting, fluorescent lamp ballasts, other electronic ballasts, and the like, which commonly use transformerless power supplies. It has been found that when load equipment which incorporates a power factor correcting circuit is connected to receive power from these AC power supply systems, instability manifested as an oscillation in the output voltage provided to the load may occur. Particularly affected are UPSs and SPSs which employ voltage regulation with feedback, either electronic or magnetic, which is derived either directly or indirectly from the output terminals. Also affected are other AC power supply systems including those which have voltage regulation or which use a ferroresonant transformer or a linear transformer with high leakage reactance in the power supply path.
Interaction between the peak currents demanded by the power factor correcting load with the voltage regulation circuitry of a UPS or SPS, or with the reactive characteristics of a transformer, can create an unstable situation in which positive feedback can produce undesirable and wide fluctuation of the amplitude of the voltage and current supplied to the power factor corrected load. The power factor correcting equipment demands current in the form of pulses which supply the energy lost by the capacitive filter to the load, and also creates a simulated current which is sinusoidal in shape and is in phase with the applied voltage. These pulses generate an additional voltage drop across the leakage reactance of the transformer. When the demand for current ceases, the voltage regulator action or the collapsing magnetic field within the transformer induces an increasing transient voltage at the output. This increasing transient voltage causes an increase of current into the filter capacitor of the power factor correcting load, triggering the action of the power factor correcting circuit and causing it to demand "correcting" current. This current causes voltage drop across the output of the regulated power supply or the transformer leakage reactance, and the cycle repeats itself. This can result in an undesirable cyclic oscillation of the AC voltage and current peaks (an amplitude modulation of the 60 Hz sinusoidal power) which, once initiated, can be difficult to stop.
Such oscillations can also occur where power is supplied from the AC power mains to a power factor correcting load through a ferroresonant or linear transformer which is not part of a voltage regulated power supply. Ferroresonant transformers, by design, have a higher leakage reactance than linear transformers, and thus in certain cases can be particularly sensitive to the effects of power factor correcting loads. Even linear transformers which have a significant leakage reactance can be similarly affected, to various degrees, depending upon the transformer design and the magnitude of the leakage reactance.
Various solutions have been sought to the instability problem. Where a transformer contributes to the problem, one approach is to substitute larger, higher power transformers, as well as different designs for the transformers. Larger transformers, which have lower leakage reactance, can minimize the problem, but are more expensive, heavier, and occupy more space than a transformer which would be properly sized to service the load if the instability problem were not present.